"if it ain't broke, don't fix it" :D
Seriously though ... let's get the nomenclature correct, first :p What you really mean to say is "cover", not "structure". Structure is the bottom composition of the lake (channels, drops, cuts, bends) .... cover is the brush, rocks, trees, weeds, & other stuff that rises off the bottom.
If there is no cover, or limited amounts ... the fish may be accustomed to free ranging. If the cover is mostly in the depths of the lake, there may not be enough oxygen to sustain them at those depths (so they stay in the upper water column). If there's cover in the shallows, then it's probably being used .... but may just be being used in the early AM hours, & during the spawn. Clear water may also be a factor, causing the fish to seek out depth for security, rather than cover. The type of baitfish present (if any) could also keep the fish out in open water.
Personally, I don't see the problem :o .... if you're catching them, trolling (or any other method), with any regularity .... why would you want to mess with fishing around cover (where you're just as likely to hook into the cover, as you are a fish)
I'm all for "don't sweat the small stuff" ... if trolling or fishing open water is the method that works, then don't worry about why it works ... just use that method, & be happy that it does work :D
... cp